A man named Rory MacPhee has just been granted England's first licence to gather and sell "sea vegetables", which include seaweed. It may come as a shock to some of the hundreds of amateur foragers living and working on the British coast that if they sell the seaweed they harvest, they're breaking the law. Time was when many people living in these islands ate seaweed every day.

Seaweed, in fact, is one of the most useful natural substances on the planet. It's existed for over one billion years, and all land plants evolved from it. At least 145 of its roughly 10,000 different species are eaten around the world. It's full of carbohydrates, proteins, minerals and vitamins, and it's often rich in iodine. Dulse, which MacPhee plans to harvest in particular, contains every trace element that human beings need.

In eastern China, seaweed is a major vegetable – though the "seaweed" you buy in your local takeaway is likely to be deep-fried cabbage. In Ireland they mash seaweed into porridge; in Hawaii they harvest and rinse it and eat it with fish. Iceland teems with free-growing ingredients: edible seaweeds were one of the few things that people could eat there in previous bleaker centuries. Laverbread is arguably Wales's greatest delicacy, a superb savoury foil to bacon and buttered toast made with the seaweed laver.

Though Indonesia produces more, Japan is probably the world's most important consumer of seaweed. They eat at least 21 species there. Nori is the flaky, crackly stuff that sticks to your tongue when you bite a sushi roll. Its annual trade is worth more than $1bn, making it the most valuable aquaculture in Japan, worth more than fish and seafood.

Another Japanese seaweed, kombu, is difficult for humans to digest, but it has nonetheless proved to be one of the most transformative ingredients of all time. In the early 20th century a Japanese chemist found that kombu was an especially rich source of monosodium glutamate – in fact, when you dry kombu, it forms little white crystals of monosodium glutamate (MSG) on its surface. MSG, of course, is now manufactured by the tonne and used across the global food industry. People say vaguely but correctly that it makes foods "taste more of themselves": it enhances the inherent savouriness of a dish, rather than seasoning it with a bitter, salty, sweet or sour note. The Japanese word for this flavour, umami, translates roughly as "delicious". It's worth noting that, contrary to its reputation, MSG is one of the safest and best-studied additives used in food. Those who claim that the MSG in Chinese takeaways makes them feel sick are more likely suffering the effects of a surfeit of cheap grease.

Processing certain red seaweeds gets you carrageenan, aka E407 – another of the most important additives in all industry. They stick it in toothpaste, shampoo, aerosol foams, shoe polish and pharmaceuticals. It winds up in ice creams, beer, pet food, soy milk and diet fizzy drinks. Carrageenan is a stabiliser and thickener. It's the only substance known to attack the cold virus directly. But while people have used it for centuries to thicken sauces, not least in Ireland and Scotland, it's probably not a good idea to eat too much of those processed foods that contain a lot of it. It certainly causes inflammation in rats, and studies on mice and guinea pigs have suggested a link between carrageenan and colon cancer.

Nonetheless, old-fashioned seaweeds are nutritious and delicious, and it's rather a shame that we abandoned them in this country. If MacPhee manages to get more Britons eating the stuff, he should be applauded.